1. Field of the Invention
Example embodiments relate to technology for controlling a bias using an integrated circuit (IC) instead of using a pilot tone, and more particularly, to technology for controlling a bias to maximize an eye output by directly monitoring an output of data, that is, eye, which differs from an existing harmonic monitoring method.
2. Related Art
An amount of data handled at various types of information technology (IT) systems, including a cloud service and a social network service (SNS), is currently on the astronomical increase. Accordingly, the demand for an optical communication interface for high speed data transmission is increasing. For the optical communication, a transmission end needs an optical modulator configured to convert an electrical signal to an optical signal. As for the optical modulator, a Mach-Zehnder modulator (MZM) and a ring modulator (RM) are currently most widely used and studied.
However, as shown in graphs 110 of FIG. 1A and graphs 120 of FIG. 1B, the optical modulators experience a significant change in a direct current (DC) bias due to a temperature. Thus, a bias control circuit for controlling the DC bias is required to have productivity in the reality.
FIG. 1A and FIG. 1B illustrate a change in a characteristic of an optical modulator according to the related art.
The graph 110 of FIG. 1A shows a change in a characteristic curve of a Mach-Zehnder-typed optical modulator over time, and the graph 120 of FIG. 1B shows a change in a characteristic curve of a ring-typed optical modulator according to a change in a temperature. Each of the two graphs 110 of FIG. 1A and 120 of FIG. 1B shows an initial curve and a curve after going through a bias change after 20 minutes. It can be known from the graphs 110 and 120 that a bias control circuit is required.
As an existing method for bias correction, disclosed is a method of finding a bias of an optical modulator using a pilot tone, for example, Korean Patent Registration No. 10-1190863 and Korean Patent Registration No. 10-0606100. This method applies a DC bias of a modulator to which a clean pilot tone is added and controls a bias using a second harmonic component among a plurality of harmonic components monitored at an output end. In a transfer function of the modulator, the linearity varies based on a bias location and sizes of even and odd harmonic components vary.
If a DC bias is controlled to minimize an amplitude of a signal acquired by monitoring the second harmonic component, biasing may be performed to a point at which the linearity of the transfer function is maximized. Since the linearity is directly used, the above method may be used based on a proposition that a point corresponding to the largest linearity is an optimal bias, such as in an MZM. In addition, the above method has some additional disadvantages. First, a clean pilot tone needs to be additionally provided from an outside. Second, a pilot tone needs to be provided using a small amplitude and a low speed of a few kHz in order to avoid the effect against a data signal. Accordingly, due to the small amplitude of the pilot tone, a sensitivity of a signal required to acquire the second harmonic component may be significantly small, which may lead to triggering a signal-to-noise ratio (SNR) issue. Also, due to a significantly low center frequency of a band-pass filter required for monitoring, integration to an IC may not be performed in the reality. As a matter of fact, an operation of completely integrating a bias control circuit on a single IC using the above method has not been reported.